JP5491500B2 - Particle mixture for producing an aluminum titanate type porous structure - Google Patents

Description

The present invention relates to a particle mixture and a method for producing a porous structure such as a catalyst support or a particle filter using the mixture. The material constituting the filter part and / or the active part is based on aluminum titanate. The ceramic material forming the main component of the ceramic filter or ceramic carrier according to the present invention is mainly composed of oxides of Al and Ti elements in the form of aluminum titanate Al ₂ TiO ₅ (Pseudobrookite) type. Mainly formed. The invention also relates to a honeycomb porous structure obtained from such a method, in particular a catalyst carrier or particle filter, in particular a catalyst carrier or particle filter used in the exhaust line of a diesel internal combustion engine. These properties are improved.

  In the following description, the use and advantages of a filter or catalyst carrier in a particular field (the field to which the present invention relates) for removing pollutants contained in exhaust gas from a gasoline internal combustion engine or a diesel internal combustion engine will be described. At present, all structures for purifying exhaust gas generally have a honeycomb structure.

  As is well known, a particle filter is exposed to a series of filtration stages (soot accumulation) and regeneration stages (soot removal) during its use. During the filtration phase, soot particles emitted from the engine are retained and deposited in the filter. During the regeneration phase, soot particles are combusted in the filter to restore filtration characteristics. Therefore, both the low temperature and high temperature mechanical strength properties of the materials making up the filter are paramount for this application. Similarly, this material can withstand temperatures that can rise well above 1000 ° C., especially if the regeneration phase is under-controlled, especially over the entire life of the equipped vehicle. It is necessary to have a structure that is sufficiently stable and durable.

At present, the filters and carriers are mainly made of porous ceramic materials, in particular silicon carbide or cordierite. Silicon carbide filters are described in, for example, Patent Documents 1 to 5. Such filters have excellent thermal conductivity and are chemically poor with porosity characteristics (especially average pore size and pore size distribution) that are ideal for applications that filter soot from internal combustion engines. It is possible to obtain an active filter structure. However, several disadvantages specific to this material still exist: the first disadvantage is the somewhat higher coefficient of thermal expansion of SiC (which makes it impossible to produce large monolithic filters ( 3 × 10 ⁻⁶ K ⁻¹ ). This often necessitates dividing the filter into a plurality of honeycomb parts and joining them together with an adhesive (for example, as described in Patent Document 3). The second drawback is the economic nature, due to the extremely high firing temperature, typically above 2100 ° C. This sintering ensures sufficient thermodynamic strength of the honeycomb structure, especially during the continuous regeneration phase of the filter. Such temperatures require the introduction of special equipment, which significantly increases the cost of the final filter obtained.

  From another perspective, cordierite filters are known and have been used for a long time due to their low cost. However, it is currently known that problems may occur with the structure, especially during under-controlled regeneration cycles where the filter may be locally exposed to temperatures above the melting point of cordierite. . The results of these hot spots can range from some loss of filter efficiency to the most severe case of its complete destruction. Moreover, the chemical inertness of cordierite is inadequate at the temperature reached during the continuous regeneration cycle, resulting in reaction with bases and other metals accumulated in the structure during the filter stage. And tend to be eroded by these. This phenomenon may cause a rapid deterioration of the properties of the structure.

  For example, such disadvantages are described in US Pat. No. 6,057,096, which is based on aluminum titanate (60-90 wt%) reinforced with mullite (10-40 wt%) to remedy those defects. Based on the proposed filter. The durability of this filter has been improved.

  However, experiments conducted by the applicant have shown that the thermal stability, mechanical strength properties, and thermodynamic strength properties appropriate to be useful directly in applications as catalyst supports or particle filters in automobile exhaust lines. Guaranteeing the performance of such structures based on aluminum titanate type porous materials, in particular, controlling the level of porosity of the finally obtained porous filter material. It was difficult.

European Patent Application Publication No. 816065 European Patent Application No. 1142619 European Patent Application Publication No. 1455923 International Publication WO 2004/090294 International Publication WO 2004/065088 International Publication No. WO2004 / 01124

  Therefore, an object of the present invention is to provide a material having a controlled porosity and which can be used as a main material for the filter part of the filter or the active part of the catalyst carrier, and a method for producing the same. It is.

  The material mainly contains or consists of an aluminum titanate type oxide material. And it has the above-mentioned characteristics that can be advantageously used in the field of catalyst carriers or particulate filter type honeycomb porous structures currently used in automobile exhaust lines.

In its most general form, the invention relates to a particle mixture mainly comprising a pseudo-plate titanite type oxide phase containing at least titanium and aluminum, or a particle mixture consisting of this oxide phase. This mixture is obtained from at least two particle size parts:
A coarse particle size portion where the median diameter d ₅₀ is _greater than 12 μm; and a fine particle size portion where the median diameter d ₅₀ is 0.5 to 3 μm.
Here, the mass ratio of the coarse portion to the fine portion is 1.5 or more and 20 or less, and the ratio of the median diameter of the coarse portion to the median diameter of the fine portion is more than 12.

For example, in the particle mixture, at least some of the particles have a main phase composed of a pseudo-plate titanite type oxide phase containing at least titanium and aluminum, and at least one secondary phase. The second phase is a phase consisting essentially of a glassy phase and / or titanium oxide TiO ₂ .

The pseudo-plate titanite type oxide phase can contain titanium, aluminum, and optionally magnesium and / or zirconium, in proportions such that the aluminum titanate type phase approximately satisfies the following formula:
(Al ₂ TiO ₅ ) _x (MgTi ₂ O ₅ ) _y (MgTiZrO ₅ ) _z
(Where 0.1 ≦ x <1; 0 <y ≦ 0.9; and z = 1−xy).

  Typically in this formula: 0.70 ≦ x <1, and 0 <y ≦ 0.3, preferably 0.80 ≦ x <1, and 0 <y ≦ 0.2.

According to one advantageous embodiment, the coarse particle size portion has a median diameter d ₅₀ of greater than 15 μm.

For example, the coarse particle size portion has a median diameter d _{50 of} less than 100 μm, preferably less than 80 μm, more preferably less than 50 μm, even less than 40 μm, even less than 38 μm, and even less than 35 μm.

  According to a preferred embodiment, the mass ratio of the coarse part to the fine part is 1.5-5, in particular 1.5-4.

  The ratio of the median diameter of the coarse part to the median diameter of the fine part is, for example, more than 15. In particular, good results have been obtained when the ratio of the median diameter of the coarse part to the median diameter of the fine part is less than 50, or even less than 45, or even less than 40. For example, this ratio can be 20 or more and 25 or less.

  All possible combinations according to the invention between the various preferred embodiments featuring the particle parts according to the invention and mixtures thereof as described above are reported so that the description is not unnecessarily burdensome. do not do. However, all possible combinations of initial and / or preferred ranges and values as described above can be envisioned and should be construed as described by the applicant within the context of this specification. It is clear that this must be done (especially a few, three or more combinations).

The invention also relates to a method for producing a catalyst support or honeycomb particle filter from a particle mixture as described above, which has the following main steps:
a) mixing the coarse and fine portions together;
b) mixing the particle mixture in the presence of a methylcellulose-type organic binder and optionally a pore-former, and an amount of water sufficient to obtain the plasticity to perform step c) c) die Extruding the unfired honeycomb structure through; and d) firing the structure at a temperature between 1300 ° C and 1800 ° C.

  The present invention also relates to a catalyst support which can be obtained by the method as described above, and which mainly contains or consists of a pseudo-plate titanium stone type oxide phase containing at least titanium and aluminum.

  Finally, the present invention is obtained by the method as described above, and the filter part mainly includes or consists of a pseudo-plate titanite type oxide phase containing at least titanium and aluminum. The present invention relates to a filter or a catalytic particle filter.

According to the first embodiment of the present invention, the porous material is composed of simple oxides of Al ₂ O ₃ , TiO ₂ , and other elements capable of forming the pseudo-plate titanite structure Al ₂ TiO ₅ . It is obtained from an oxide, for example an oxide in the form of a solid solution. Such materials are typically magnesium oxide, silicon oxide, zirconium oxide, iron oxide, or oxides of other elements. This mixture is sintered. That is, the mixture is heated to a temperature at which simple oxides can react to form sintered particles containing at least one main phase having an Al ₂ TiO ₅ type structure.

Alternatively, according to the invention, instead of the simple oxide, any precursor of the oxide can also be used, for example in the form of carbonates, hydroxides or other organometallics of the above elements. The term “precursor” means a material that decomposes into a simple oxide corresponding to the most previous stage of heat treatment, ie heat treatment typically at temperatures below 1000 ° C., below 800 ° C., and even below 500 ° C. Understood. As described above, the precursor mixture is sintered. That is, the mixture is heated to a temperature at which the precursor can react so as to form particles having at least one main phase having an Al ₂ TiO ₅ type structure.

According to another possible embodiment of the invention, the material according to the invention comprises the oxides Al ₂ O ₃ , TiO ₂ and optionally MgO, ZrO ₂ , SiO ₂ or other oxides (or These precursors) are synthesized from particles obtained by pre-melting.

  For example, according to the present invention, the particles are obtained by a fused casting method, which allows mass production with high yield and very good price / performance ratio.

A continuous melt casting step for the production of particles is for example as follows:
a) mixing the raw materials to form a starting quantity;
b) melting the starting amount to obtain a melt;
c) cooling the melt, thereby completely solidifying the melt (this cooling can be done quickly, for example in less than 3 minutes); and d) optionally, the solids are mixed with the particle mixture Crushing step to get.

  Of course, without departing from the scope of the present invention, any composition for producing molten cast particles can be used, provided that the composition of the starting amount allows obtaining particles having a composition that matches the composition of the particles of the present invention. Other conventional or known methods can be used.

  In step b), an electric arc furnace is preferably used, but any known furnace, such as an induction furnace, a plasma furnace, can be envisaged if the starting quantity is completely melted. Calcination is preferably carried out under inert conditions, for example under argon or oxidizing conditions, preferably at atmospheric pressure.

  In step c), cooling is not necessary but is preferably carried out quickly, ie in such a way that the molten liquid is completely solidified in less than 3 minutes. Preferably, the cooling is done from a cast in a CS mold as described in US Pat. No. 3,993,119 or by a quenching operation.

  In step d), the solid is ground using conventional techniques until the particle size of the present invention is obtained.

One method for producing the above structure from the initial particle mixture of the present invention is generally as follows:
First, the particles obtained by sintering or melt casting as described above are mixed. In the present invention, the molten cast particles have been pulverized to have an appropriate particle size within the meaning of the present invention. In a manner well known in the art, this process typically involves mixing an initial particle mixture with a methylcellulose type organic binder and a pore former (eg, starch, graphite, polyethylene, PMMA, etc.). And stepwise adding water until the necessary plasticity is obtained to allow the step of extruding the honeycomb structure.

  For example, during the first step, the particle mixture is mixed with 1-30 wt% of at least one pore former selected according to the desired pore size, and at least one organic plasticizer and / or organic binder As well as water.

  Mixing results in a homogeneous product in the form of a paste. Extruding this product through a suitably shaped die makes it possible to obtain a honeycomb shaped monolith using well known techniques. For example, the method can include drying the resulting monolith. During the drying step, the resulting green ceramic monolith is typically subjected to sufficient time to reduce the content of chemically unbound water to less than 1 wt% by microwave drying or heat drying. Can be dried. If it is desired to obtain a particle filter, the method may further include the step of plugging every other flow path at each end of the monolith.

  The step of firing the monolith on which the filter part is based on aluminum titanate is in principle carried out at a temperature above 1300 ° C. but not exceeding 1800 ° C., preferably not exceeding 1750 ° C. This temperature is adjusted in particular by other phases and / or oxides present in the porous material. Typically, during the firing step, the monolith structure is heated to a temperature between 1300 ° C. and 1600 ° C. in an atmosphere having oxygen or an inert gas.

  One advantage of the present invention is that unlike SiC filters (as described above), there is no need for segmentation and the possibility of obtaining a very large size monolith structure. However, although not preferred, in one embodiment, the method optionally includes the step of assembling the monolith into a filter structure that is assembled using known techniques, such as those described in EP-A-816065. May be included.

  The filter structure according to the invention or the structure made of porous ceramic material is preferably of the honeycomb type. This generally has a suitable porosity of 20 to 65%, preferably 30 to 50%, and this average pore diameter is ideally 10 to 20 μm.

  Such filter structures typically have many adjacent conduits or channels with mutually parallel axes separated by a wall formed by a porous material.

  In a particle filter, the conduit is closed by a plug at one or the other end so as to define an inlet chamber open to the inlet surface and an outlet chamber open to the outlet surface, thus Gas is allowed to pass through the porous wall.

The invention also relates to a filter or catalyst support obtained by depositing (preferably impregnating) at least one active catalyst phase which is supported or preferably not supported from a structure as described above. Its active catalyst phase typically comprises at least one noble metal, for example Pt, Rh, and / or Pd, and optionally oxides, such as _{CeO 2, ZrO 2, CeO 2} -ZrO 2. The catalyst carrier also has a honeycomb structure, but the conduit is not blocked by plugs and the catalyst is deposited in the pores of the flow path.

  The invention and its advantages will be better understood by reading the following non-limiting examples. In this example, all percentages are given in weight percent unless otherwise noted.

In all examples, all percentages are given in wt%. Test specimens were prepared from the following ingredients:
About 40 wt% alumina (AR75 ™, marketed by Pechiney) with a purity greater than 99.5% and a median diameter d _{50 of} 90 μm;
About 50 wt% rutile TiO ₂ (sold by Europ Minerals) containing> 95 wt% TiO ₂ and about 1% zirconia and having a median diameter d ₅₀ of about 120 μm;
About 5 wt% SiO ₂ (sold by SIFRACO) having a purity of greater than 99.5% and a median diameter d _{50 of} 208 μm; and a purity of greater than 98% and greater than 80% About 4 wt% MgO (sold by Nedmag) where the particles have a diameter of 0.25 to 1 mm.

The mixture of initial reactive oxides was melted in an electric arc furnace in the atmosphere at oxidizing electrical conditions. The molten mixture was cast into a CS mold for quick cooling. The resulting product was crushed and sieved to obtain powders with various particle size portions. More precisely, grinding and sieving were carried out under conditions to finally obtain four particle size parts, ie the following particle size parts:
A particle size portion characterized by a median diameter d _{50 of} about 30 μm, denoted by the term “coarse portion” in the present invention;
A particle size portion characterized by a median diameter d _{50 of} about 16 μm, denoted by the term “coarse portion” in the present invention;
A particle size portion characterized by a median diameter d _{50 of} about 1.5 μm, denoted by the term “fine portion” in the present invention; and Part of the particle size characterized by the diameter d ₅₀ .

Within the context of this description, the median diameter d ₅₀ is the diameter of a particle measured by liquid phase sedimentation (sedigraphy), where the smaller particle is present at 50% of the population volume. Means.

  Microprobe analysis showed that all particles of the molten phase thus obtained had the following composition at the oxide weight percent.

The composition and properties of the phase present in the particles were also analyzed, and the results of the analysis are shown in Table 2. Based on these results, it was possible to estimate the weight percent of each phase by calculation. Based on the data obtained, pseudoplate titanite (Al ₂ TiO ₅ ) _x (MgTi ₂ O ₅ ) _y , which is in the form of an Al ₂ TiO ₅ type solid solution incorporating the elements Mg and Zr, in particular for the main phase AMTZ. The values of x and y corresponding to the general formula of (MgTiZrO ₅ ) _z structure can also be determined.

In Table 2:
1) The crystalline phase present in the refractory product was characterized by X-ray diffraction. In Table 2, AMTZ represents (Al ₂ TiO ₅ ) _x (MgTi ₂ O ₅ ) _y (MgTiZrO ₅ ) _z- type solid solution (where Z = 1−xy), P2 is the second Corresponding to the subphase, Ps indicates the presence of an additional silicate phase.
2) The chemical composition of the various phases, expressed as% by weight based on the oxide, was measured by X-ray fluorescence.

  In order to study the properties and advantages of porous structures based on the materials obtained by the method of the invention in application as catalyst supports and / or particle filters, those mentioned above and / or criteria of the invention Various test specimens were prepared from the resulting molten particle portions for comparison with those not satisfying.

  A rod of porous ceramic material was typically obtained in the following manner: One or more particle size portions were mixed with 4 wt% methylcellulose type organic binder and 15 wt% pore former. Water was added and mixed until a homogenous paste with plasticity was obtained that would allow the specimen to be extruded in the form of a 8 mm x 6 mm cross section and 70 mm long rod. The bar was then sintered at 1450 ° C. for 4 hours.

  For these specimens, in order to estimate the value of the porous material in the “particle filter” application, the bending failure modulus MOR (modulus of rupture), the porous properties and the shrinkage during sintering are thus obtained. Measurements were made on porous rods.

  Typical porosity characterization (total open porosity and median pore diameter) was measured by a well-known technique, high-pressure mercury porosimetry, using a 9500 porosimeter (Micromeritics). .

  The shrinkage during sintering represents the dimensional change of the test piece after sintering at 1450 ° C. More precisely, in the present invention, the term “shrinkage during sintering” means the average along each of the two dimensions of the cross section of the material, which lasts to a low temperature, ie to a temperature below 400 ° C., in particular to room temperature. Is understood to mean a decrease in The shrinkage values shown in Table 4 correspond to the average shrinkage for the two dimensions, expressed as a percentage of the initial dimension of the bar before sintering for each of its dimensions. This property is extremely important in order to evaluate the feasibility of the manufacturing method of the porous structure. In particular, large sintering shrinkage means that honeycombs formed from this material will be subject to great difficulty, especially with respect to obtaining with acceptable reproducibility during industrial scale-up. This is particularly difficult for structures with dimensional characteristics that can guarantee sufficient precision to allow the materials to be used without any difficulty, especially in the exhaust lines of automobiles.

  The flexural failure factor (MOR) was measured at room temperature in a three-point bending test on a previously obtained porous rod of 60 mm × 6 mm × 8 mm.

  All the results obtained are given in Table 4.

  In addition, a honeycomb monolith was obtained by extrusion through a die using the same mixture of particle size portions. The dimensions of the monolith are given in Table 3.

  The resulting green monolith was cut and tested in order to check the uniformity of material distribution within the honeycomb extruded structure.

  The ease of extrusion was also evaluated by measuring the flow pressure required for extrusion through a 2 mm diameter spaghetti die of the material.

  Depending on the above two parameters (ie uniformity of material distribution within the honeycomb extrusion structure and flow pressure to extrude spaghetti), the possibility (or impossibility) of extruding the particle mixture was determined. . The results of the “pushability” analysis of the mixture are given in Table 4.

  In Table 4, Examples 1 and 2 are given for comparison. In these two examples, the material was provided only by the coarse initial mixture. In such a case, as shown in Table 4, the porous honeycomb structure could not be extruded acceptably. Furthermore, the MOR values obtained for the bars are low.

  Example 7 given for comparison uses two parts, a coarse part and a fine part. If the ratio of the median diameter of the coarse part and the fine part is less than 12, there is a problem of formation, and particularly Indicates that extrusion problems will occur.

  The results given in Table 3 show that Examples 3-6 according to the present invention make the material difficult or unusable in such applications for use as a porous catalyst structure and / or filter in the exhaust line. To the extent that it does not, it indicates that none of its essential properties results in materials and products that do not degrade.

Example 8 illustrates an embodiment using three particle size parts (medium size part plus medium size part according to the invention). In particular, Table 4 shows that only the particle mixture according to the invention as defined in the claims makes it possible to obtain the following materials:
1) a material that can be easily formed for the above applications, i.e. a material that can be easily extruded into a honeycomb structure; and 2) mechanical strength, porosity, and stability (shrinkage) properties suitable for the above applications. Material that combines.

Claims (9)

The oxide phase of pseudobrookite titanite type containing at least titanium and aluminum to a particle mixture consisting mainly containing or the oxide phase, Re obtained et the following at least two grain diameter portion:
A coarse particle size portion where the median diameter d ₅₀ is _greater than 12 μm; and a fine particle size portion where the median diameter d ₅₀ is 0.5 to 3 μm;
(Here, the mass ratio of the coarse portion to the fine portion is 1.5 or more and 20 or less, and the ratio of the median diameter of the coarse portion to the median diameter of the fine portion is more than 12) ,And
The pseudoplate titanite type oxide phase contains titanium, aluminum, and magnesium and / or zirconium in a proportion such that the aluminum titanate type phase approximately satisfies the following formula:
(Al ₂ TiO ₅ ) _x (MgTi ₂ O ₅ ) _y (MgTiZrO ₅ ) _1-xy
(Where 0.1 ≦ x <1; 0 <y ≦ 0.9; and z = 1−xy). The particle mixture of claim 1, wherein the coarse particle size portion has a median diameter d ₅₀ of greater than 15 μm. 3. The particle mixture according to claim 1, wherein the coarse particle size portion has a median diameter d _{50 of} less than 100 μm, preferably less than 80 μm, more preferably less than 50 μm. The particle mixture as described in any one of Claims 1-3 whose mass ratio with respect to the said fine part of the said coarse part is 1.5-5. The particle mixture according to any one of claims 1 to 4 , wherein the ratio of the median diameter of the coarse part to the median diameter of the fine part is more than 15, preferably 20 or more and 25 or less. A method for producing a catalyst support or honeycomb particle filter from the particle mixture according to any one of claims 1 to 5 , comprising the following main steps:
a) mixing the coarse portion and the fine portion together;
b) mixing the particle mixture in the presence of a methylcellulose-type organic binder and optionally a pore-former, and an amount of water sufficient to obtain the plasticity to perform step c) c) through a die Extruding the unfired honeycomb structure; and d) firing the structure at a temperature of 1300 ° C to 1800 ° C. Catalyst support obtainable by the process according to 請 Motomeko 6. Particulate filter can be obtained by the method described in 請 Motomeko 6. A catalytic particle filter obtainable by the method according to claim 6.